olive oil
coconut oil
nut butters/ground nuts (not whole nuts)
eggs
oily fish (tuna, mackerel, salmon, trout, sardines, sprats) - off the top of my head I think boys are limited to 2 portions of oily fish per week. I can check if you need me to.
Yes milk is still the primary source of food although between now and 12 months the solids will increase. Is there a reason he actually needs to gain weight though?